Graphitized carbon is currently the anode of choice for commercial Li-ion batteries due to its stable cycling behavior and low cost. However, a disadvantage of using carbon as a battery material is its inherently low theoretical charging capacity of 372 mAh/g. In particular, FIG. 1 shows a chart of charging capacities expressed as volumetric (mAh/l) and gravimetric (mAh/g) values for a variety of materials. As can be seen from the chart, alternative materials such as silicon (Si) and germanium (Ge) have much higher charging capacities than carbon, affording the potential to make lithium ion batteries based upon Si or Ge materials that are significantly lighter and longer lasting than carbon-based batteries. Although Si exhibits the highest known charging capacity of any anode material at 4200 mAh/g, germanium, either in pure form or as a component of a SiGe alloy, is also a promising candidate for lithium ion batteries for several reasons. For example, germanium has the second highest theoretical gravimetric charging capacity of any anode material at 1625 mAh/g. Germanium also has a higher electrical conductivity than Si, which facilitates a faster rate of electron transfer to a current collector in a battery. Moreover, the lithium diffusion coefficient for Ge is 400 times greater than Si at room temperature. Both of these properties are important for high rate applications, since the electrochemical reaction (see Eq. 1) that takes place in lithium ion batteries is limited by electron and lithium ion transport.Li4.4Si4.4Li++Si+4.4e−  (1)
In addition, unlike Si, Ge does not form a stable oxide. In particular, the native oxide layer that may be formed on a silicon surface not only consumes some of the active anode material (Si), but also acts as a diffusion barrier to Li ions. Accordingly, anode materials based at least in part of Ge may have superior performance due to less anode consumption and higher lithium ion diffusion rates. Moreover, although Ge forms the same Li22X5 phase as Si during battery charging, Ge exhibits a lower volumetric expansion (270%) during this process, which results in more stable cycling and less capacity fade after a given number of charging cycles. Ge is also completely miscible with Si, so that Si and Ge can be combined in any ratio to form a stable alloy. This property makes it possible to mix Si and Ge to form a SiGe alloy of any desired composition to take advantage of the high theoretical capacity of Si on the one hand and the beneficial kinetic properties of Ge on the other hand.
While Si and Ge are attractive as anode candidates in lithium ion batteries based upon their charging capacities, conventional thin film Si and Ge anodes suffer from physical integrity issues, such as pulverization, which may result from repeated cycling through charging and discharging processes. FIG. 2a depicts a charging process of an anode material, such as Si. When the anode 20 is charged, lithium ions 22 flow from the 24 cathode and into the anode 20. After charging, the lithium ions 22 may form a Li22Si5 phase, as illustrated in FIG. 2b. At the stage illustrated in FIG. 2b, the creation of this Li22Si5 phase causes the Si to expand up to 400%. Upon discharge (shown at its beginning stages in FIG. 2b), the lithium ions diffuse out of the anode 20, which may result in the disappearance of the Li22Si5 phase and cause the remaining Si atoms to contract back to a smaller volume and reconstitute in a Si phase. This process of Li22Si5 phase formation and destruction, together with the concomitant expansion and contraction of the anode layer from the phase changes typically causes the anode to separate from the current collector over time, resulting in a drop in charging capacity.
FIG. 3 shows a microstructure of a thin film Si anode portion 30 after being subjected to repeated charging/discharging cycles. The microstructure depicted in FIG. 3 is based upon a low magnification secondary electron microscope (SEM) image of a thin film Si anode portion 30. As is evident, the lithiation process, in which lithium diffuses into the silicon anode during charging and diffuses out of the silicon anode during discharge, induces sufficient stress to cause the film to segregate into Si islands 32, which eventually may separate completely from the substrate 34. High cycling rates also increase the rate of delamination of the deposited film. Moreover, although the volumetric expansion is less for germanium, thin films of Ge when used as lithium ion battery anode suffer from the same failure mechanism. In this manner, capacity fade due to delamination is typical for lithium ion battery structures made using Si or Ge thin films as anodes.
Accordingly, before Si and Ge can be integrated into commercial lithium ion battery technology, the problem of capacity fade should be addressed. Recently, several researchers have tested anodically etched porous Si as an anode material, with promising results. The porous microstructure of the silicon is believed to provide room for expansion and contraction that occurs during respective charging and discharging steps, which may help the silicon to maintain contact with the current collector after repeated cycling.
Porous silicon is a material that has received increased attention especially due to its excellent properties in energy storage application. However, in spite of extensive study for over 30 years, there has been little improvement in the fabrication technique for porous silicon since its invention. The anodization process is typically expensive and complex, thereby making anodized porous silicon less attractive for high volume applications such as lithium ion batteries. Therefore, it will be apparent that improved apparatus and methods are desirable.